port away from GTK2

This needs a port to GTK3 or at least away from GTK2. This was first reported in Debian as #885282 and will likely block inclusion of the project in the next Debian release if nothing is done.

Possible options:

• GI/GTK3 - i'm angry at them for breaking compat last time, so maybe not. plus accessing keysyms was hard in GTK2 and maybe also there
• Kivy - huge engine, but may do everything we need. cross-platform compat (including android) as bonus
• QT4/QT5 - another good alternative, but QT is big as well...
• wxpython - not working with Python3 yet...

All have been tested as working in Wayland, for what it's worth.